Field of Application
The present invention relates to a monitoring system for detecting leakage current of a DC power source and/or a short-circuit failure of relays in an electrical system supplying power from the DC power source to an electrical apparatus.
Description of Related Technology
A monitoring system for detecting leakage current in a DC power source is known, as described in Japanese patent No. 3781289 (designated in the following as reference 1). A pair of electric power leads are connected to the DC power source, and power is supplied through the electric power leads to an electrical apparatus. a relay is installed in the electric power leads, controlled for connecting/disconnecting the DC power source to/from the electrical apparatus. A signal generator section which generates an AC signal and a measurement section which measures a voltage value of the AC signal are coupled to one of the electric power leads, at a position between the DC power source and the relay of that electric power lead. When a leakage current flows in the DC power source, the AC signal flows to ground, so that the value of the AC signal voltage measured by the measurement section becomes lowered. With such a monitoring system, when the measured AC signal voltage falls below a predetermined value, it is judged that leakage current is flowing from the DC power source.
The relay contains switches, which may experience a short-circuit failure. For example the switch contacts may become welded together, or a drive circuit of the relay may become defective, so that the switch may remain held in the on state even if it is attempted to be set to the off state. It is proposed to use the above-described AC signal to perform monitoring for detecting such a short-circuit failure. The background of that proposal is as follows. The switches are respectively connected in the pair of electric power leads. In the prior art, to perform monitoring to detect short-circuit failure of such a switch, a first one of the switches (and only that switch) is controlled to be set in the on state. If at that time the other switch (second one of the two switches) has a short-circuit failure, a current will flow through a capacitor that is connected in parallel with the electrical apparatus, so that the measured AC voltage value will increase. Hence, by judging whether or not the capacitor voltage rises, a decision can be made as to whether the second switch has a short-circuit failure. If it is judged that the second switch does not have a short-circuit failure, then next, the second switch(and only that switch) is controlled to be set in the on state. If at that time the first switch has a short-circuit failure, then a current will flow through the capacitor, so that the measured AC voltage value will increase. Hence, by judging whether or not the capacitor voltage rises, a decision can be made as to whether the first switch has a short-circuit failure.
In the prior art, to perform such switch short-circuit failure monitoring, it is necessary to operate the pair of switches respectively separately. However if only a single electromagnetic coil is used, the pair of switches cannot be operated respectively separately, but can only be operated to the on and off states concurrently. Hence, it is necessary for the relays to employ two electromagnetic coils, for operating the switches to the on and off states. As a result, the problem arises that the manufacturing cost of such a relay becomes increased. Hence there is a requirement for avoiding such an increase in manufacturing cost, by using only single electromagnetic coil.
Thus it has been envisaged that the above-described AC signal might be used to monitor for switch short-circuit failure, even in the case of relays in which the respective switches cannot be operated separately.
That is to say, a low value of resistance could be established between ground and respective positions on each electric power lead between the corresponding switch and the electrical apparatus. In that case if there is a short-circuit failure of a switch, the AC signal produced by the signal generating section would flow through the switch and pass via the corresponding electric power lead to the electrical apparatus, and would flow to ground from the electric power lead via the low resistance. By monitoring the level of the AC signal, a decision could be made as to whether a switch has a short-circuit failure, i.e., based on whether or not the AC signal voltage has decreased from a normal level. In that way, short-circuit failure monitoring could be performed without executing on/off operation of the switches, i.e., with both switches set to the off state. Thus, it could be envisaged that such a method would be applicable even in the case of a relay in which the two switches cannot be operated respectively separately.
However if such a method were attempted, it would be difficult to judge whether short-circuit failure of a switch has actually occurred. That is, since it is necessary to sufficiently insulate each electric power lead from ground, the resistance value between the electric power lead and ground must be substantially high. Thus even if there is a short-circuit failure of a switch, the level of current which flows to ground must be small, so that the measured voltage of the AC signal will not fall substantially.